1. Field of the Invention
The present invention relates to photomasks used in a photolithography process in manufacturing of semiconductor devices or the like.
2. Description of the Related Art
FIGS. 3A and 3B show a typical conventional example of the photomasks, in which mask patterns 2 are formed on a quartz substrate 1, where each mask pattern is made of a light blocking (or shielding) material which is metallic and conductive. More specifically, each mask pattern 2 is made of one or more layers of chromic light blocking material such as Cr (chromium) or chromic oxide, which are stacked on each other.
In such a photomask, a pellicle 3 may be provided on one face of the photomask, on which the mask patterns 2 are formed, or on both faces of the photomask. The pellicle 3 is provided for preventing foreign particles from landing on the photomask (when the photomask is used). Such foreign particles on the photomask may produce a defective pattern in the exposure process.
The pellicle 3 consists of a pellicle frame 4 which is typically attached to a peripheral area around the mask patterns 2, and a pellicle film 5 which is adhered to the pellicle frame 4.
In this structure, even when foreign particles land on the pellicle 3, if the particle size of the foreign particles is not very large (i.e., smaller than a specific size), a considerable distance can be secured between the foreign particles and the top faces of the mask patterns 2. Therefore, in the exposure process, an optical image of the foreign particles is defocused, so that the shapes of the foreign particles are not transferred.
In the above-explained photomask, as shown in FIGS. 3A and 3B, the mask patterns 2 are independently provided on the quartz substrate 1, and thus the mask patterns 2 are also electrically isolated from each other. Therefore, electrostatic damage may occur due to electrification of the photomask itself, static electricity charged from the outside, or the like.
There are photomasks having long service life, which are used for long periods of 1 year or more. Regarding photomasks used for long periods, in particular, when the mask patterns 2 are made of metallic material or the like, the photomask is generally used for a long time and the mask patterns 2 may be damaged by radiation energy from an exposure apparatus (e.g., a stepper).
In order to solve the above problems, the following measures have been conventionally taken.    1. The operator who treats the photomask puts on a grounding band, antistatic shoes, and the like, so as to avoid being electrically charged.    2. An ionizer is employed in the place where the photomask is treated, so as to avoid the photomask being electrically charged.    3. An expensive mask inspection system is employed so as to perform acceptance inspection, sampling inspection (regularly performed), and the like, so as to confirm the quality of the photomask and to maintain required quality of the products.
Although the above-explained measures are performed as minimum requirements, problems such as electrostatic damage of the photomask and damage of the pattern cannot be solved at present.